How to Check Wheel Offset Before You Buy

You’re going from 8.5J ET35 to 9.5J ET25. The wheel is an inch wider and the offset is 10 mm lower, so it sticks out more. By how much? And does the inner edge move too?

Both edges move, in opposite amounts, and the inner one is the one that hits something expensive.

What offset actually is

Offset — marked as ET on the wheel, from the German Einpresstiefe — is the distance from the wheel’s mounting face to its centreline, in millimetres.

Positive offset puts the mounting face outboard of the centreline, pulling the wheel inward. Lower or negative offset pushes it outward.

Width changes both edges. Add an inch of width and, with the same offset, half an inch goes outward and half an inch inward. Changing offset moves the whole wheel in one direction.

So:

inner change = (Δwidth ÷ 2) + (ET_new − ET_old)

outer change = (Δwidth ÷ 2) − (ET_new − ET_old)

From 8.5J ET35 to 9.5J ET25: half an inch is 12.7 mm each side, and 10 mm less offset pushes everything outward. That’s 22.7 mm further out at the face and 2.7 mm further in at the back.

The outer number gets all the attention because it’s visible. The inner number is what contacts a strut, a brake line or the inner arch liner.

Tyre width is the real constraint

Rim measurements aren’t what touches anything. A 245-section tyre on a 9.5J rim has a section width wider than the rim, and it bulges further under load and on turn-in.

Clearance also changes through the suspension travel and steering lock. A wheel that clears at rest can foul at full lock over a bump — which is the classic “it was fine on the driveway” fitment failure.

Why people get stuck here

• Only the poke is considered. Inner clearance is the one that damages things.
• Rim width used instead of tyre width. The tyre is what makes contact.
• Static measurement. Clearance at rest doesn’t predict clearance at full lock and full compression.
• Spacers as a general fix. They move the wheel outward only, which cures inner clearance and worsens outer.

Common mistakes to avoid

• Checking clearance with the car parked and the wheels straight.
• Fitting spacers without correct-length bolts and a hub-centric design — this is a safety-critical component, not a shim.
• Assuming what fits a friend’s car fits yours, when suspension and arch geometry differ between trims.
• Ignoring how a lower ride height changes arch clearance completely.
• Buying wheels before checking brake caliper clearance, which the offset calculation doesn’t cover at all.

How to do it with Wheel Offset Calculator

The Wheel Offset Calculator shows both edges moving, which is what fitment planning needs.

1. Enter your current wheel width and offset — J width in inches, ET in millimetres.
2. Enter the proposed wheel’s figures.
3. Read both the inner and outer movement.
4. Add tyre section width, and check clearance at full lock and full compression, not at rest.

Other vehicle calculators are in the tools directory.

Frequently asked questions

Does a lower ET always mean more poke?

For the same width, yes — lower offset moves the wheel outboard. But changing width moves both edges at once, so a wider wheel with the same ET pokes out and moves inboard simultaneously.

Does tyre width matter for clearance?

Very much. Tyre section width exceeds rim width and bulges under load, so the tyre usually contacts an arch or a strut before the rim does.

Can spacers fix a fitment problem?

They move the wheel outward only. That cures inner clearance and worsens outer. They also need correct-length bolts and a hub-centric design — treat them as a safety-critical part, not an accessory.

Final thought

Calculate both edges, then check clearance at full lock with the suspension compressed. Every fitment surprise happens in a condition nobody tested on the driveway.

Try the free Wheel Offset Calculator